Advances in Sucker Control for Sustainable European Hazelnut (Corylus avellana L.) Cultivation

European hazelnut (Corylus avellana L.) is a shrub native to temperate zones of the northern hemisphere, and it is the most important species among the Corylus genus, mainly due to its high kernel demand from the confectionery industry. Its spontaneous habitus is characterized by a bushy shape, formed by numerous lignified stems generated by seasonal emission of suckers, which develop from adventitious buds inserted in the collar of stems, or more generally from the stump. Despite the agronomic role of suckers, which are often used to replace diseased, old, or poorly oriented branches, they compete with the plant for water and nutrient uptake, negatively influencing its growth and yield. In addition to promoting mechanical applications during the hazelnut orchard management, sucker removal is a required agronomic operation that must be carried out yearly during the growing season, making this practice expensive and time consuming, especially when performed manually. To date, there are several techniques for hazelnut sucker management, and their application depends on several factors, such as the size of the farm, model of cultivation (conventional or organic), soil orography, and plant training system. This review discusses the most widespread methods applied for sucker control, including manual, mechanical, physical, and chemical control (flame and steam), use of non-suckering rootstocks, disbudding, mulching, nitrogen solution applications, and new automatized control techniques recently proposed at the experimental level, analyzing their advantages and disadvantages.